This is a randomized, double-blind, placebo-controlled clinical trial designed to evaluate the effects of high-dose short-chain fatty acid (SCFA) supplementation on the gut microbiome and host metabolome in stable kidney transplant recipients. Participants will be randomly assigned to receive either 1000 mg of sodium butyrate per day or placebo for a duration of 12 weeks. Comprehensive profiling of the serum and urinary metabolome, along with analysis of the gut microbiome composition and diversity, will be conducted at three time points: baseline, after the intervention period (week 12). The biochemical parameters and the level of tacrolimus will be also examined.

Inclusion Criteria:

• Age ≥ 18 years
• Primary kidney transplantation (living or dead donor)

Stable kidney transplant recipients (≥ 6 months post-transplantation)

Stable graft function defined as eGFR ≥ 30 mL/min/1.73 m² with no significant change (>15%) in the last 3 months

No episodes of acute rejection within the last 6 months

On stable immunosuppressive therapy for at least 3 months

Ability to provide written informed consent

Willingness and ability to provide stool, urine, and blood samples at specified time points

Exclusion Criteria:

• Use of antibiotics, probiotics, or prebiotics within 4 weeks prior to enrollment

Active gastrointestinal disease (e.g., Crohn's disease, ulcerative colitis, celiac disease)

Severe intestinal motility disorders or chronic diarrhea

Advanced liver disease (Child-Pugh C)

Active infection or systemic inflammatory disease requiring treatment

Uncontrolled diabetes mellitus (HbA1c > 9%)

Known allergy or intolerance to SCFA or ingredients in the supplement

Participation in another interventional clinical trial within the past 30 days

Pregnancy or breastfeeding

Hospitalization within 30 days prior to enrollment

Any condition which, in the opinion of the investigator, may compromise the safety of the participant or the integrity of the study data